When plasma-enhanced chemical vapor deposition chambers are used repeatedly for thin film deposition, a layer of the material to be deposited gradually builds up on the interior surfaces of the chamber. The chamber must therefore be cleaned periodically. This can be done by disassembling the system, but there are many advantages to cleaning the chamber in situ by etching the deposits from the interior surfaces. For systems used to deposit dielectric films, perfluoro compounds (i.e., C.sub.2 F.sub.6 and other gases containing carbon and fluorine) have been used for in situ cleaning. It is believed, however, that the use of perfluoro compounds (PFC's) may generate gases that contribute to global warming, and the major U.S. integrated circuit manufacturers have entered into a memorandum of understanding with the U.S. Environmental Protection Agency that calls for voluntary efforts to reduce the levels of PFC generation.
One way of reducing the level of PFC's is to switch to fluorinated source gases such as NF.sub.3, either alone or mixed with He or other inert gases. These gases have an inherently higher etch rate and are therefore more efficient in cleaning the chambers. One obstacle to doing this in a plasma-enhanced chemical vapor deposition (PECVD) chamber, however, is that NF.sub.3 generates an electronegative plasma which is more difficult to stabilize than the plasma created by PFC's. Electronegative plasmas have stability problems because the impedance of an electronegative plasma increases as it becomes more electronegative. This causes less power to be delivered to the plasma. If this trend is sufficiently fast, the plasma will be extinguished. After charge neutralization occurs, the plasma will be re-ignited and will proceed through this cycle again. In a multi-electrode PECVD system the power will not be distributed equally among the electrodes because the power will go preferentially to the lower impedance electrodes.
Unless the plasma is stabilized, oscillations can occur in the plasma and between electrodes such that controlled power delivery is difficult at best and the impedance matching network cannot achieve a match. Stabilizing gases have been used to mitigate this situation, but these gases may reduce the maximum etch rate achievable. With an unstable plasma, arcing can also cause serious damage to the chamber.
Thus there is a clear need for a way of stabilizing an NF.sub.3 containing plasma in a PECVD chamber.